1. Field of the Invention
The present invention relates to a novel CD44-like protein receptor. In particular, isolated nucleic acid molecules are provided encoding the CD44-like protein. CD44-like protein polypeptides are also provided, as are screening methods for identifying agonists and antagonists capable of enhancing or inhibiting CD44-like protein-mediated signaling. The invention further concerns therapeutic methods for treating diseases associated with processes mediated by CD44-like protein signaling.
2. Background Information
CD44 (also known as Pgp-1, Hermes-3, HCAM, ECMR III) is a widely expressed glycoprotein with a molecular weight of 85 to 90 kDa (Haynes et al., Immunol. Today 10:423-428 (1989)). Immunological studies have shown that CD44 is involved in a diverse range of biological functions such as lymphocyte binding to high endothelial venules (Jalkanen et al., J. Cell. Biol. 105:983-990 (1987)), lymphopoiesis (Miyake et al., J. Exp. Med 171:477-488 (1990)) and activation of leukocytes (Webb et al., Science 249:1295-1297 (1990)). CD44 has also been shown to play a role in extracellular matrix binding, cell migration, lymphocyte activation, lymphocyte homing, and proliferation of bronchial smooth muscle cell (Herrlich et al., Immunology Today, 14(8):395-399, (1993); Lesley et al., Immunology, 54:271-355, (1993); Lazzar et al., Journal of Experimental Medicine, 180:807-816 (1994)). A splice variant of CD44, CD44-V6, has been shown to play a role in tumor cell metastasis (Gunthert et al., Cell, 65:13-24 (1991)). The CD44 cDNA sequence has revealed a domain of some 90 amino acids near the N-terminus bearing a significant similarity to the link and core proteins in proteoglycan (Goldstein et al., Cell 56:1063-1072 (1989)), leading to the determination that hyaluronate, a major component of the extacellular matrix, is a ligand for CD44 (Miyake et al., J. Exp. Med 172:69-75 (1990); Aruffo et al., Cell 61:1303-1313 (1990)). Fibronectin and collagen type I and VI have also been shown to interact with CD44 (Carter et al., J. Biol. Chem. 263:4193-4201 (1988)).
CD44 is a widely distributed heterogenous population of cell surface adhesion molecules. The CD44 gene has twenty exons, ten of which encode the sequence for standard CD44. An additional ten variable exons are inserted by alternative splicing.
Although the standard 85 kDa CD44 is broadly expressed by many different types of cells, the expression of CD44 variants is rather limited. Arch et al. have demonstrated a transient expression of a CD44 variant, V6, in leukocytes from animals after allogeneic immunization (Arch et al., Science 257:682-685 (1992)), indicating a possible role for this variant in mediating leukocyte trafficking.
Initially, the heterogeneity of CD44 was thought to be due to post-translation modification, especially the addition of chondroitin sulfate, which gives rise to a higher molecular weight from of about 200 kDa (Jalkanen et al., J. Immunol. 141:1615-1623 (1998)). The finding of a protein isoform in rats capable of conferring metastatic potential to nonmetastatic cells established a new role for CD44, namely, regulating cell migration (Gunthert et al., Cell 65:13-24 (1991)).
Interestingly, several recent studies have ascribed unique functional activities to certain alternatively spliced isoforms, raising the possibility that the inclusion of additional peptide sequences within the extracellular domain of CD44 may alter the ligand-binding specificity of the molecule. As an example, CD44H, the major CD44 isoform present on resting hemopoietic cells, and CD44R1, a differentially expressed isoform containing a 132 amino acid insertion coded by the alternatively spliced exons v8-v10, can bind both immobilized and soluble hyaluronan when transfected into the CD44-negative murine lymphoma cell line TIL1. However, only the expression of CD44R1 can induce these cells to homotypically aggregate. This homotypic aggregation may be mediated by the adhesive interaction between a determinant encoded by the insertion region present in CD44R1 and a common region present in both CD44R1 and CD44H.
Different isoforms of CD44 are often preferentially expressed by certain cell types. A widely expressed CD44 isoform is the "standard" or "hematopoietic" CD44 (CD44s) molecule, which is encoded by exons 1-5, 15-17 and 19 of the CD44 gene. This 85-95 kDa molecule is the principle CD44 isoform on hematopoietic cells and lymphocytes. Larger CD44 variants that contain different combinations of alternatively spliced exons are preferentially expressed on epithelial cells, but they can also be found on activated lymphocytes and high grade malignant lymphomas.
From the above, it will be understood by one of ordinary skill in the art that the identification of additional CD44 variants and CD44 homologues is of great importance.